The prior art is, for example, illustrate din U.S. Pat. Nos. 2,985,589 and 4,498,991 which constitute the standard texts respectively for simulated counter-current and simulated co-current modes. It is explicitly stated therein that each inlet and outlet stream shifts simultaneously and periodically with all the others. In U.S. Pat. No. 5,144,590, again each stream is simultaneously shifted with the others, period by period, except for one in particular which takes two different positions during the shift period T. The prior art thus teaches a unique period T between two successive shifts for each stream, the product of this period and the number of column sections n or independent beds, n-T, constituting the cycle time.
Such a conventional simulated moving bed separation process is basically operated with either 4 or 5 different flow rates at the recycling pump; these flow rates correspond to the flow rates in the four or five zones in the process. In industry, for practical reasons involving purges of lines carrying different streams, processes are also known which operate with six or even seven different flow rates at this pump. A number of problems arise from the fact that during each cycle, the recycling pump changes flow rates as frequently as there are zones in the process:
The pump must be tailored to the highest flow rate: it is thus too strong for the others. PA1 At each flow rate change, the pressure profile in the columns undergoes a sudden and unavoidable variation which among others, disturbs the flow rate of the pressure controlled output stream (raffinate). PA1 At each change, the passage from one flow rate to the next is not instantaneous: in an industrial unit, for example, 2 seconds for passage from one flow rate to another indicates good regulation: thus for about 10 seconds in a cycle lasting on the order of 2000 seconds, the flow rate is poorly defined. PA1 The flow rate in each zone must be constantly monitored to ensure it is constant whatever the position of the zones with respect to the recycling pump. Taking the imperfections in measuring and regulating flow rates of the recycle, inlet and outlet streams into account, it is difficult to achieve a flow rate fluctuation of less than 2%. PA1 The pump is tailored to a single flow rate, preferably lower than the highest of the flow rates described above. PA1 The pressure profiles are no longer perturbed when the configuration is changed. PA1 Since the flow rate remains substantially constant, it is of little importance that regulation of the flow rate is only slow. PA1 The flow rates in each zone do not remain constant during a cycle but it is sufficient that the differences between these flow rates (which represent the inlet and outlet flow rates) do remain constant for the internal consistency of the system to be strictly ensured. PA1 The total cycle time is the same for each stream: each inflow and outflow stream must traverse the whole of the circuit in the same time as the others. As a corollary, each section is in an identical position with respect to the streams at the end of the same period, namely the cycle time. PA1 In the first embodiment described above, each stream must be controlled independently from the others; either using one on-off valve per section and per stream (total n.multidot.z valves) or using a valve with n positions per stream, i.e., a total of z valves. PA1 If in the first embodiment the connection points of one particular stream sometimes displace faster than those of the following stream (in the direction of flow), the number of sections contained in this zone in the initial configuration must be at least 2, otherwise the two streams will be found between the two same sections at particular instants during the cycle. More precisely, the system comprises at least 2z-2 distinct sections in the whole of the system. PA1 Q.sub.k represents the flow rate in zone k in the conventional simulated mobile bed (Q.sub.1, Q.sub.2, Q.sub.3, Q.sub.4 for a simulated mobile bed with 4 zones), PA1 T represents the shift period for the conventional simulated mobile bed (a single value), PA1 Q.sub.k.sup.p represents the flow rate in zone k in the constant recycle simulated mobile bed when the pump is in zone p, PA1 T.sub.i+q.sup.j represents the instant when the introduced or extracted stream shifts from the outlet to the j.sup.th section to the outlet to the following section, where: PA1 for the first, at least z.sup.2 different terms, PA1 for the second, at least n.times.z different terms. PA1 In noway does it predict the form of the adsorption isotherms: in any conventional simulated mobile bed, the volume leaving each zone, Q.sub.k xT, remains substantially constant throughout the cycle. PA1 Zone 1 is located between the solvent injection point and the extract extraction point, and in this zone the flow rate is Q.sub.1 =Q.sub.4 +S for the conventional simulated mobile bed. PA1 Zone 2 is located between the extract extraction point and the feed injection point, and in this zone the flow rate is Q.sub.2 =Q.sub.4 +S-E for the conventional simulated mobile bed. PA1 Zone 3 is located between the feed injection point and the raffinate extraction point, and in this zone the flow rate is Q.sub.3 =Q.sub.4 +R for the conventional simulated mobile bed. PA1 Zone 4 is located between the raffinate extraction point and the solvent injection point, and in this zone the flow rate is Q.sub.4 for the conventional simulated mobile bed. PA1 The constant flow rate applied at the recycling pump to the constant recycle simulated mobile bed is Q.sub.c. PA1 The pump passes from zone 4 to zone 3 at T.sub.2.sup.7 and not T.sub.2.sup.3 since at this instant the raffinate outlet passes from the outlet to C.sub.7 to the outlet to C.sub.8. PA1 The pump passes from zone 3 to zone 2 at T.sub.4.sup.7 and not T.sub.4.sup.5 since the feed injection passes from the inlet to C.sub.8 to the inlet to C.sub.1 at the same instant. PA1 The pump effectively passes from zone 2 to zone 1 at T.sub.6.sup.7. PA1 In this particular case, the average recirculation flow rate for the conventional simulated mobile bed is ##EQU4## The value of Q.sub.c can be arbitrarily fixed (provided that it is greater than the solvent flow rate). However: PA1 If Q.sub.c is substantially lower than the average flow rate Q.sub.M of the simulated mobile bed for which the operation is desired to be improved, the cycle time becomes very large, and certain T.sub.j.sup.i will become much stronger than others, then a very large number of sections is required in certain zones in order to avoid the disappearance of these zones. In addition, since the flow rates Q.sub.k.sup.p become very different from flow rates Q.sub.k, it is no longer possible to assume that the transfer resistance remains identical to the conventional simulated mobile bed and the constant recycle simulated mobile bed. PA1 If Q.sub.c is much greater than the average flow rate Q.sub.M of the conventional simulated mobile bed, the pressure drops in the unit will increase considerably, and the height of the equivalent theoretical plate will increase, so the length of the column must be increased if a given separation quality is to be retained. PA1 In practice, advantageously, if the operation of an existing unit is to be improved, Q.sub.c is selected such that Q.sub.c is between Q.sub.4 +S and the highest of the two values Q.sub.4 or S, Q.sub.4 being the lowest of the recycle flow rates in a conventional simulated mobile bed: in fact the recycling pump is tailored to transport a maximum flow rate Q.sub.1 =Q.sub.4 +S and a minimum flow rate Q.sub.4. In the case of a new unit, there is greater freedom. In general, the quantity of fixed phase is minimized and in general, an interval of less than 20 seconds is not used for the interval between shifts otherwise reasonable precision is lost for this value (the movements of the valves controlling the introduction or extraction circuits are not instantaneous). PA1 test for EQU Q.sub.1 &gt;Q.sub.3 &gt;Q.sub.2 &gt;Q.sub.5 &gt;Q.sub.4 or Q.sub.1 &gt;Q.sub.3 &gt;Q.sub.2 &gt;Q.sub.4 &gt;Q.sub.5 PA1 A sequencing table equivalent to Table 3 is drawn up, also a time-flow rate table for the conventional simulated mobile bed and the constant recycle simulated mobile bed, respectively analogous to Tables 1 and 2. PA1 5 series of equations (1 per zone) are then drawn up, giving T.sub.j.sup.i as a function of Q.sub.4, S, E, RI, Ch, R and T, where RI represents the internal reflux flow rate. PA1 The five series of equations will lead to one and the same explicit equation which is fifth degree in Q.sub.c and the physical value of Q.sub.c will be close to Q.sub.M. If the system with five zones is written as follows: ##STR1## the following 5th degree equation can be derived: EQU .alpha.Q.sub.c.sup.5 +.beta.Q.sub.c.sup.4 +.gamma.Q.sub.c.sup.3 +.delta.Q.sub.c.sup.2 +.epsilon.Q.sub.c +.phi.=0 PA1 Q.sub.c &gt;Q.sub.4 (2), i.e., .vertline.Q.sub.4 .vertline..gtoreq..vertline.Q.sub.c since, following our convention, PA1 Q.sub.4 =-.vertline.Q.sub.4 .vertline. and thus .vertline.Q.sub.4 .vertline..gtoreq..vertline.Q.sub.2 .vertline.&gt;.vertline.Q.sub.1 .vertline..gtoreq..vertline.Q.sub.3 .vertline.. From Table 2, for which all the flow rates in the zones remain oriented in the same direction ,the inequality .vertline.Q.sub.c .vertline..gtoreq.R must also be satisfied. PA1 the pump passes from zone 4 to zone 3 at T.sub.2.sup.8 and not T.sub.2.sup.6 PA1 The pump passes from zone 3 to zone 2 at T.sub.4.sup.8 PA1 The pump passes from zone 2 to zone 1 at T.sub.6.sup.8 and not t.sub.6.sup.2. PA1 point no 4 raised in the SCC case remains valid with the only difference being that the absolute value of Q.sub.c must this time be greater than the raffinate flow rate which is the highest of the inlet and outlet flow rates.
Prior art document WO 93/22022 describes a simulated mobile bed system with a constant recycle flow rate: the recycling pump is displaced in the same way as the inlet and outlet points and thus the latter is always located in the same zone and hence its flow rate remains constant. However, this system requires that each section of the simulated mobile bed is in a separate column and, in addition to its connections to the introduction and extraction circuits and with the preceding and following sections, each section is connected to the intake and to the output of the recycling pump and is isolatable, thus of course rendering the system far more expensive.
The prior art is also illustrated by the patents EP-A-0577079 and U.S. Pat. No. 3,268,605.